The present invention relates to the document processing arts. The invention finds particular application in conjunction with a method and apparatus for imbedding hidden information in documents that are rendered using error diffusion, and will be described with particular reference thereto.
Images captured in electronic form can be easily copied, distributed, and even altered. It can sometimes be valuable to identify the source and integrity of an image. One approach is to include information within the image which is normally not visible, but which can be detected if desired. This information can identify the owner of the image or might reveal image alterations. This hidden information is typically referred to as a digital watermark. It is desirable to have the digital watermark present, not only in the electronic image, but also in prints or output copies of the electronic image. One would like the watermark to be preserved even if the print or output copy is itself copied or scanned.
Image information, be it color or black and white, is commonly derived by scanning, initially at least, in a gray level format containing a large number of gray intensity levels, such as 256 levels for color, which is usually not reproducible on standard output devices such as binary printers, etc. Digital halftoning converts image information comprising a large number of gray scaled pixel values to a reduced number of gray scaled pixel values in order that image information can be rendered for display or hardcopy (i.e. printed) output.
Error diffusion is another technique that converts gray level images to binary or a lesser level of gray while attempting to preserve an overall gray density of the images. Error diffusion attempts to maintain overall gray density by making the conversion from gray pixels to binary or other level pixels on a pixel-by-pixel basis. The procedure examines each pixel value with respect to a threshold. If the pixel value is darker than the threshold a black dot is imaged, otherwise white is printed. The difference between the value printed (black or white) and the actual pixel value of is calculated and distributed among the neighboring pixels that have yet to be rendered.
Accordingly, it has been considered desirable to develop a new and improved method and apparatus for imbedding hidden information in output documents and copies or scanned images of output documents using an error diffusion rendering technique, which meets the above-stated needs and overcomes the foregoing difficulties and others while providing better and more advantageous results.
In accordance with one aspect of the present invention, a method for imbedding a watermark in an output copy is disclosed. The method includes the steps of a) providing an image representation defined by a plurality of pixels; b) comparing a first pixel intensity value to a threshold value; c) rendering a dot on an output copy, the dot having a second intensity value based on the comparison of step b); d) distributing a difference between the first and second intensity values among a plurality of pixels located proximate the first pixel; e) modifying the threshold value with a biasing value and a watermark value; and f) repeating steps a)-e) for each of the plurality of pixels.
In accordance with another aspect of the present invention, a method for imbedding a watermark in an output copy is disclosed. The method includes the steps of a) providing an image representation defined by a plurality of pixels; b) rendering a pixel based on a threshold value, a biasing value, and a watermark value; c) modifying the threshold value by varying at least one of the biasing value and the watermark value to influence whether a dot is generated in the output copy; and d) repeating steps b) and c) for each of the plurality of pixels.
In accordance with yet another aspect of the present invention, a method for imbedding a watermark in an output copy is disclosed. The method includes the steps of a) storing an image representation defined by a plurality of pixels; b) rendering the output copy using an error diffusion routine that parses through the plurality of stored pixels; c) modifying the error diffusion routine in step b) with a biasing pattern that influences where on the output copy each of the plurality of pixels is rendered; and d) further modifying the error diffusion routine in step b) with a watermark pattern that modifies the biasing pattern for pixels that are positioned within the watermark.
One advantage of the present invention is the provision of a method for imbedding hidden information in images that are rendered using error diffusion.
Another advantage of the present invention is the provision of a method for introducing a bias in the dot placements of a rendering algorithm and then modulating that bias with a pattern to be hidden.
Yet another advantage of the present invention is the provision of a method for imbedding a watermark in an output document that cannot be destroyed by subsequent copying and/or scanning of the document.
A still further advantage of the present invention is the provision of a method for authenticating a digital image by simply superimposing a biasing pattern over the image.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiment.